Termination of transmission lines using simultaneously enabled pull-up and pull-down circuits

ABSTRACT

An information handling system includes a plurality of transmission lines coupled together at one end and having a characteristic impedance, a driver circuit coupled to one of the transmission lines, and at least one receiver circuit coupled to the transmission lines. At least one driver circuit includes pull-up and pull-down circuits each having output resistances. The parallel combination of the pull-up and pull-down resistances matches the characteristic impedance of the transmission line. When the driver is used to terminate the transmission line, the pull-up and pull down circuits are enabled at approximately the same time, thus terminating the line. On-chip terminators may alternatively be used for termination, the terminators either including pull-up and pull-down circuits substantially similar to those described for the drivers, or may include a single resistance tied to a voltage other than an upper or lower rail. A method for managing the information handling system described above includes terminating each node by causing either an on-chip terminator or a driver to go into a terminating mode, thus causing the termination of the line using an output resistance that matches the characteristic impedance of the transmission line.

BACKGROUND ART Cross-Reference to Related Applications

This application is a con of Ser. No. 08/881,927 filed Jun. 25, 1997.

This application relates to co-pending U.S. patent application Ser. No. 08/881,939, filed on Jun. 25, 1997, entitled Impedance Control Circuit and naming Sai V. Vishwanthaiah, Jonathan E. Starr, and Alexander D. Taylor as inventors, the application being incorporated herein by reference in its entirety.

This application relates to co-pending U.S. patent application Ser. No. 08/881,925, filed on Jun. 25, 1997, entitled Broadly Distributed Termination For Buses Using Switched Terminator Logic and naming Jonathan E. Starr as inventor, the application being incorporated herein by reference in its entirety.

This application relates to co-pending U.S. patent application Ser. No. 08/881,927, filed on Jun. 25, 1997, entitled Method of Broadly Distributed Termination For Buses Using Switched Terminators and naming Jonathan E. Starr as inventor, the application being incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to buses and more particularly to termination of buses for use in information processing systems.

2. Description of the Related Art

In computer and information processing systems, various integrated circuit chips communicate digitally with each other over a common bus. The signal frequency at which this communication occurs can limit the performance of the overall system. Thus, the higher the communication frequency, the better. The maximum frequency at which a system communicates is a function not only of the time that it takes for the electromagnetic wavefronts to propagate on the bus from one chip to another, but also of the time required for the signals to settle to levels that can be recognized reliably at the receiving bus nodes as being high or low, referred to as the settling time.

The length of the settling time is a function of the amount of reflection and ringing that occurs on the transmission line. The more effective the termination of a bus system, the smaller the effects of reflection and ringing in the system and the shorter the overall settling time of the signal.

SUMMARY OF THE INVENTION

An information handling system includes a plurality of transmission lines coupled together at one end and having a characteristic impedance, a driver circuit coupled to one of the transmission lines, and at least one receiver circuit coupled to the transmission lines. At least one driver circuit includes pull-up and pull-down circuits each having output resistances. The parallel combination of the pull-up and pull-down resistances matches the characteristic impedance of the transmission line. When the driver is used to terminate the transmission line, the pull-up and pull down circuits are enabled at approximately the same time, thus terminating the line. On-chip terminators may alternatively be used for termination, the terminators either including pull-up and pull-down circuits substantially similar to those described for the drivers, or may include a single resistance tied to a voltage other than an upper or lower rail. A method for managing the information handling system described above includes terminating each node by causing either an on-chip terminator or a driver to go into a terminating mode, thus causing the termination of the line using an output resistance that matches the characteristic impedance of the transmission line.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.

FIG. 1 is a block diagram of an information handling system having driver, receiver, and termination circuits in accordance with the present invention.

FIG. 2 is a block diagram of an information handling system according to the present invention.

FIGS. 3A, 3B, 3C and 3D show schematic block diagrams of a variety of bit elements of the driver circuits in accordance with the present invention.

FIG. 4 is a block diagram of an alternate embodiment of the present invention.

FIG. 5A is a signal response curve for a system as depicted in FIG. 2 showing a voltage overshoot condition.

FIG. 5B is a signal response curve for a system such as is depicted in FIG. 4.

FIGS. 6A, 6B and 6C represent three alternate embodiment systems employing the present invention.

DETAILED DESCRIPTION OF ONE EMBODIMENT

Those of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons having the benefit of this disclosure.

The use of the same reference symbols in different figures indicates similar or identical items.

FIG. 1 is a block diagram of an information handling system having driver, receiver, and termination circuits in accordance with the present invention.

Referring to FIG. 1, information handling system 100 includes a plurality of devices 102 which communicate with each other over transmission lines 104.

Each component 102 includes a driver circuit 106, a receiver circuit 108 and an optional terminator circuit 110. The output resistance of each of driver circuits 106 is controlled using impedance control circuitry as is known in the art. One such means of controlling the output resistance of a driver is disclosed in U.S. patent application Ser. No. 08/881,939 filed on Jun. 25, 1997, entitled Impedance Control Circuit and Sai V. Vishwanthaiah, Jonathan E. Starr, and Alexander D. Taylor as inventors, the application being incorporated herein by reference in its entirety.

Herein, a node is defined as an endpoint of a transmission line which is not directly coupled to another transmission line. It may be coupled to one or more of a driver, receiver, or terminator circuit, or a combination of such circuits. For example, the system of FIG. 1 has three nodes.

A driver may comprise a pull-up circuit, a pull-down circuit, or both. A pull-up circuit, when enabled, couples the output node to the upper power rail through a controlled output resistance; when disabled it presents a high impedance to the output node. A pull-down circuit, when enabled, couples the output node to the lower power rail through a controlled output resistance; when disabled it presents a high impedance to the output node. A driver transmits signals into the transmission line by selectively enabling and disabling its pull-up or pull-down circuits at the proper times.

In one embodiment, components 102 include terminators 110 which include an output resistance which is matched to the characteristic impedance of the transmission lines 104 and which is coupled to the transmission line 104 whenever the corresponding driver circuit 106 is not driving signals. When a terminator 110 is enabled, the corresponding driver circuit 106 provides a high impedance to the transmission line.

In this disclosure, when an output resistance is “matched” to the characteristic impedance of a transmission line, it is intended that the output resistance be within about ten percent of the characteristic impedance of the transmission line.

In a second embodiment, terminator 110 is not present. Rather, the output resistance of the driver itself is used to terminate the transmission line. That is, when not driving signals, the driver presents a terminating resistance to the line which is approximately equal to the impedance of the transmission line.

Whether an information handling system has a separate terminator circuit or instead incorporates the termination resistances into the driver circuitry, the effect on signals arriving at the end of the node is the same.

FIG. 2 is a block diagram of an information handling system according to the present invention.

In the system of FIG. 2, an embodiment is described wherein the termination resistances are incorporated into the driver circuitry. That is, the driver output resistance is used to terminate the transmission line. Those of ordinary skill in the art who are familiar with the information contained herein would be able to practice either the FIG. 2 embodiment or an alternate embodiment which includes separate terminators 110 instead of incorporating the termination resistances into the drivers.

In the FIG. 2 embodiment, two distinct configurations are possible. In the first configuration, each non-driving node is connected to a corresponding driver whose pull-up circuit is enabled and acts to terminate the corresponding transmission line. In this first configuration, R_(PU), the output resistance of the pull-up circuit, is approximately equal to the transmission line impedance and R_(PD), the output resistance of the pull-down circuit, is approximately equal to a value computed using a formula to be described below.

In the second configuration of FIG. 2, each non-driving node is connected to a corresponding driver whose pull-down circuit is enabled and acts to terminate the corresponding transmission line. In this second configuration, R_(PU), the output resistance of the pull-up circuit, is approximately equal to a value computed using a formula to be described below and R_(PD), the output resistance of the pull-down circuit, is approximately equal to the transmission line impedance.

In both configurations, the termination resistance is approximately equal to the transmission line impedance, and the termination resistance is tied to the transmission line whenever the driver is not driving the line. When a node is in a driving configuration, the output resistance of the driver at any given moment is determined by either R_(PU) which is the output resistance of the driver's pull-up circuit when it is enabled, or R_(PD) which is the output resistance of the driver's pull-down circuit when it is enabled, or it is high impedance if neither the pull-up nor the pull-down circuit is enabled.

Referring to FIG. 2, each driver circuit 106 of information handling system 100 includes a pull-up circuit 202 and a pull-down circuit 204 coupled to transmission lines 214, transmission lines 104 having a characteristic impedance of Z_(O).

When a node is driving signals onto transmission line 104, the driver 106 of the node either pulls the signal towards the upper power rail, pulls the signal towards the lower power rail or presents a high impedance to the output node, whichever is appropriate to send the desired signal. When a node is not driving, the pull-up portion (or pull-down portion, as appropriate) of the driver remains active, thereby providing a terminating resistance which is matched to the impedance of the transmission line.

Because the impedance at each non-driving node is matched to the impedance of the transmission line (due to the terminating resistance at the driver circuit present at a non-driving node), signals arriving at the non-driving node terminate without reflecting at that node. This condition is highly desirable because the settling time of the signal voltage level at the non-driving node is a critical parameter relating to bus performance. The net effect of the absence of reflections from the stub ends is a reduced overall settling time when compared to systems without termination at each non-driving node.

For the description which follows, it is assumed that any required termination resistance is supplied by a driver output resistance which is tied to the upper rail. Thus, R_(PU), the output resistance of a driver in the pull-up configuration, is approximately equal to the transmission line impedance.

In the system of FIG. 2, the high signal voltage seen at a non-driving node is V_(dd). V_(dd) is the voltage of the upper power rail, and V_(ss) is the voltage of the lower power rail. The low signal voltage seen at a non-driving node is V_(dd)−V_(swing) where V_(swing) is given by

 V _(swing)=(V _(dd) −V _(ss))([Z _(O)/(n−1)]/[{Z _(O)/(n−1)}+R])  (R.1)

where R is the value of the pull-down resistance (shown as R_(PD) in FIG. 2) and where n is the number of nodes in the information handling system 100. In the present invention, each node has a terminator or means for termination attached to it. The receivers are capable of resolving the swing.

Solving for R gives

R=[Z _(O)([(V _(dd) −V _(ss))/V _(swing)]−1)]/(n−1)  (R.2)

allowing for a particular pull-down resistance to be chosen for a desired V_(swing).

As an example, in the system of FIG. 2, assume that a voltage swing of (V_(dd) −V _(ss))/2 is desired. Thus, pull down driver circuit 204 includes a pull-down resistance that is substantially equal to (i.e. within 10% of) $\begin{matrix} {R = {{\left\lbrack {Z_{0}\left( {\left\lbrack {\left( {V_{dd} - V_{ss}} \right)/V_{swing}} \right\rbrack - 1} \right)} \right\rbrack/\left( {n - 1} \right)}\quad \left( {{from}\quad R{.2}} \right)}} \\ {= {\left\lbrack {Z_{0}\left( {\left\lbrack {{\left( {V_{dd} - V_{ss}} \right)/\left( {V_{dd} - V_{ss}} \right)}/2} \right\rbrack - 1} \right)} \right\rbrack \left( {n - 1} \right)}} \\ {= {{Z_{0}\left( {2 - 1} \right)}/\left( {n - 1} \right)}} \\ {= {Z_{0}/\left( {n - 1} \right)}} \end{matrix}$

where n is the number of nodes in the information handling system 100.

Because the impedance at each non-driving node is matched to the impedance of the transmission line (due to the terminating resistance at the drivers located at those nodes), signals arriving at the non-driving node terminate without reflection. This condition is highly desirable because the settling time of the non-driving node is a critical parameter relating to bus performance. The net effect of the absence of reflections from the stub ends is a reduced overall settling time when compared to systems without termination at each non-driving node.

In the FIG. 2 configuration, by having the pull-up resistors on chip, the pull-up resistance at the driving node is switched off when the driver is pulling low. Accordingly, this system advantageously consumes less overall current and power than a system that has an off-chip pull-up resistor that is always drawing current.

When the termination of a node takes place outside component 102, the terminator is separated from the non-driving node by some distance along a transmission line and thus undesirable parasitics are introduced in the connection to the termination resistor. Also, because of the separation such a system can have reflections from the intersection of the stubs of the transmission line.

When the termination is within each component, the termination resistance is placed right at the receiver, thereby reducing reflections and ringing. Thus, providing the termination within each component 102 improves signal integrity when compared to terminating a node outside component 102.

Those of ordinary skill in the art will readily recognize that although the embodiment of FIG. 2 described herein relates to a system having the termination resistances tied to the upper rail, an alternate embodiment is contemplated which instead has the termination resistances tied to the lower rail. In this alternate embodiment, the pull-down resistance of each driver is approximately equal to the impedance of the transmission line and the pull-up resistance is approximately equal to the value of R given by relationship (R.2).

In an embodiment having separate terminators 110, the output resistance of the terminator is approximately equal to the impedance of the transmission line, and that resistance is connected to or disconnected from the transmission line, depending on whether the particular node is receiving or driving respectively. In this embodiment, the driver output at a non-driving node is high impedance when the node is in a receive condition.

Referring to FIGS. 3A, 3B, 3C and 3D, the pull-up and pull-down elements of driver circuit 110 may be of a variety of configurations. For example, as shown in FIG. 3A, the driver element may be a PMOS transistor. Also for example, as shown in FIG. 3B, the driver element may be the parallel combination of a PMOS transistor and an NMOS transistor. With this parallel combination, it is the resistance of the parallel combination that would be equal to the desired bit element resistance. Also for example, as shown in FIG. 3C, the driver element may be an NMOS transistor. Also for example, as shown in FIG. 3D, the driver element may be the parallel combination of two NMOS transistors. In a preferred embodiment, the pull-up circuit element includes the parallel combination of the PMOS transistor and the NMOS transistor and the pull-down element includes the parallel combination of two NMOS transistors. It will be appreciated that a driver circuit may have other circuitry that contributes to the overall pull-up and pull-down resistance of the driver.

FIG. 4 is a block diagram of an alternate embodiment of the present invention.

Referring to FIG. 4, system 400 comprises drivers 402, 404 and 406 individually coupled to resistance elements 408, 410, and 412 through transmission lines 414, 416, and 418, with each transmission line having a characteristic impedance Z_(O) and each resistance element having a resistance of R_(T). Drivers 402, 404, and 406 may include the example configuration of FIG. 2 where the pull-up circuit has an output resistance which approximately equals the characteristic impedance of the line, and the pull-down circuit has an output resistance which is inversely proportional to the number of nodes, or may instead comprise the reverse configuration described herein. Also, the system of FIG. 4 may use separate terminators as previously described in relation to FIG. 2.

To prevent signal reflections, the impedance looking into the resistive network from one of the transmission lines must match the impedance of the transmission line. For example, signals propagating from driver 402 on the transmission line 414 will not suffer reflections at the interface between line 414 and resistor 408 if the net impedance seen looking into resistor 408 at this interface is equal to the impedance of transmission line 414.

The impedance seen looking into resistor 408, 410, or 412 from the corresponding transmission line 414, 416, or 418 is

Z _(network) =R _(T)+[(R _(T) +Z _(O))/(n−1)]  (R.3)

Where n represents the number of nodes in the system.

In order to prevent reflections,

Z _(network) =Z _(O)  (R.4)

In order to match impedances, relationship (R.4) is substituted into relationship (R.3), resulting in

 R _(T) =Z _(O)×[(n−2)/n]  (R.5)

Thus, in the three-node system of FIG. 4, each resistor in the resistor network has a value of $\begin{matrix} {R_{T} = {Z_{0} \times \left\lbrack {\left( {n - 2} \right)/n} \right\rbrack}} \\ {= {Z_{0} \times \left\lbrack {\left( {3 - 2} \right)/3} \right\rbrack}} \\ {= {{Z_{0}/3}\quad {ohms}}} \end{matrix}$

Thus, a signal propagating away from any of drivers 402, 404, or 406 will see, looking into the resistor network, a net impedance of

Z _(network) =R _(T)+0.5×(R _(T) +Z _(O))=Z _(O)/3+2Z _(O)/3=Z _(O)

In the system of FIG. 4, the voltage swing at a non-driving node is given by

 V _(swing)=(V _(dd) −V _(ss))(Z _(O)/[(R+Z _(O))(n−1)])  (R.6)

where R is the output resistance of the driver pull-down circuit if the terminating resistances at non-driving nodes are coupled to the upper power rail; otherwise, R is the output resistance of the driver pull-up circuit if the terminating resistances at non-driving nodes are coupled to the lower power rail.

Using the previously described example configuration of FIG. 2 with the resistor network of FIG. 4, the value of the pull-down resistance is determined by solving relationship (R.6) for R, giving

R=Z _(O)([(V _(dd) −V _(ss))−V_(swing)(n−1)]/[V _(swing)(n−1)  (R.7)

In systems such as the system of FIG. 2 (lacking a resistive network such as that seen in FIG. 4), a signal propagating from a driver towards the common point 214 will see an impedance at the common point which is not matched to the impedance of the transmission line. Therefore, in those systems, a reflection would occur at common connection point 214. The magnitude of that reflection increases as the impedance mismatch increases. In systems employing the embodiment of FIG. 4, the impedance seen at the connection point between the resistor and the transmission line by a signal propagating from a node to common connection point 420 is equal to the impedance of the transmission line, so no reflection occurs. Since no reflection occurs, a second signal may be sent sooner than otherwise possible, thus increasing signaling frequency of the system.

A significant benefit resulting from using the FIG. 4 embodiment of the present invention is that signals may be pipelined. Pipelining is when a signal is launched on a signal conductor prior to the most recently launched signal arriving at a receiving end. In some systems, including the system of FIG. 2, in order to transmit a signal, it may be necessary to wait until the magnitudes of the reflections on the driver stub resulting from a previously launched signal reflecting from the junction have diminished enough to not interfere with subsequently launched signals. Using the system of FIG. 4, there are no reflections. Therefore a signal may be launched by a driver prior to the time a previously launched signal has arrived at the receiving ends.

A second benefit of the FIG. 4 embodiment is that the voltage present at any given time on the lines will not exceed the driving voltage, V_(dd).

FIG. 5A is a signal response curve for a system as depicted in FIG. 2 showing a voltage overshoot condition. FIG. 5B is a signal response curve for a system such as is depicted in FIG. 4.

Both signal response curves show the voltage at the driving and receiving ends where V_(dd)=1.5V, V_(ss)=0V, the transmission line impedance is 50 ohms, the pull-down resistance is 25 ohms, and the pull-up resistance is 50 ohms.

Referring to FIG. 5A, from time t₀ to just prior to time t₁, the driver is pulling low. While the driver, assumed to be chip 1 of FIG. 2, is pulling low, there is a 25 ohm pull-down resistance in series with a parallel combination of the 50 ohm pull-up resistance of chip 2 of FIG. 2, and a 50 ohm pull-up resistance of chip 3 of FIG. 2. Therefore, the current is V_(dd)/R=1.5V/50 ohms=30 ma. The initial voltage at all driver and receiver ends is V_(dd)/2.

At approximately time t₁, the driver no longer pulls low, and is in the transition stage to pulling high. At this time, the 30 ma current flow stops, giving a ΔI of 30ma. Since Z_(O)=50 ohms, there is a ΔV of (Z_(O))(ΔI)=1.5V. Now, the total voltage at the driving end is the sum of the initial voltage plus ΔV=0.75V+1.5V=2.25V. However, when the 50 ohm pull-up element turns on, there is a 1:1 voltage divider between the pull-up resistance and the line, resulting in a divider between 1.5V and 2.25V, resulting in a total line voltage of 1.875V. Thus, the total voltage on the line exceeds V_(dd).

Between time t₁, and just after time t₃, the voltage on the line overshoots V_(dd), and could cause damage to driving elements that are not protected against such damage.

At approximately time t₂, the driven signal arrives at junction 214. Due to the impedance mismatch there, a negative reflection of the signal propagates back towards the driver. It is the reflected signal which, upon arriving back at the driver, brings the voltage at the driver end down from the overshoot condition seen at approximately times t₁to t₃ to V_(DD) at approximately time t₄.

Although the example signal response curve for FIG. 5A pertains to a system having a total of three nodes, the curve of FIG. 5A also generally applies to systems having more nodes than three. The primary difference in the curve for many nodes would be the actual overshoot voltage.

FIG. 5B utilizes the configuration described with respect to FIG. 4. As shown, this new configuration does not have an overshoot condition which would otherwise damage components that are not sturdy enough to tolerate a voltage overshoot without becoming damaged.

Referring to FIG. 5B, from time t₀ to just prior to time t₁, the driver is pulling low. It can be shown that the driver current is given by

I _(driver) =V _(dd)/(R+Z _(O))  (R.8)

The voltage out of the driver at time t₀ is simply the current multiplied by the driver resistance R, giving

V _(driver) =[V _(dd)/(R+Z _(O))]R  (R.9)

Thus, when the driver stops pulling low, the change in current is given by relationship (R.8). The change in voltage in the system is $\begin{matrix} \begin{matrix} {{\Delta \quad V} = {Z_{0}\left( {\Delta \quad I} \right)}} \\ {= {Z_{0}\left\lbrack {V_{dd}/\left( {R + Z_{0}} \right)} \right\rbrack}} \end{matrix} & \text{(R.10)} \end{matrix}$

The voltage after the driver stops pulling low is merely the sum of the original voltage and the change in voltage (R.9 and R.10), giving $\begin{matrix} {= {{\left\lbrack {V_{dd}/\left( {R + Z_{0}} \right)} \right\rbrack R} + {Z_{0}\left\lbrack {V_{dd}/\left( {R + Z_{0}} \right)} \right\rbrack}}} \\ {= {\left( {R + Z_{0}} \right)\left\lbrack {V_{dd}/\left( {R + Z_{0}} \right)} \right\rbrack}} \\ {= V_{dd}} \end{matrix}$

showing that there is no overshoot with the system of FIG. 4.

Although the example signal response curve for FIG. 5B pertains to a system having a total of three nodes, the curve of FIG. 5B also generally applies to systems having more nodes than three.

FIGS. 6A, 6B and 6C represent three alternate embodiment systems employing the present invention.

In the following description of systems employing the present invention, it is intended that the phrase “cluster node” refer to a plurality of nodes having transmission lines connected at a common point. The transmission lines directly connected to a node are cluster node transmission lines, and all other transmission lines are system transmission lines.

Referring to FIG. 6A, system 600 comprises two clusters 602 and 604 coupled together by system transmission line 606. Cluster 602 comprises nodes 608 and 610 connected to cluster transmission lines 612 and 614 respectively. In this specification, the only intended distinction between a cluster transmission line and a system transmission line is that a system transmission line is connected on both ends to transmission lines. A cluster transmission line is connected to a transmission line on one end, and a node at the other. As previously defined, a node may comprise a driver, a receiver and a driver, terminator and a driver, a receiver and a terminator, or a receiver, terminator, and driver.

There is no requirement that nodes 608 and 610 be configured identically. Rather, it is contemplated that nodes will be configured to meet the unique system needs at that node. For example, one node in a cluster may comprise a receiver and a terminator, and a different node in the same cluster may similarly comprise a receiver and a terminator, or alternatively a receiver and a driver, or just a driver. Although a node having a terminator by itself provides no receive or drive capability, a cluster employing such a node would also work.

Cluster node 604 is shown having two nodes 616 and 618 which may be configured as previously described.

Referring to FIG. 6B, system 620 includes clusters 622, 624, and 626, each cluster having one or more nodes such as nodes 628, 630, 632, 634, and 636. System transmission lines 638 and 640 are provided to couple the three clusters together. One alternative to this configuration would be to eliminate system transmission line 638, thus combining clusters 622 and 624 into a single cluster having three nodes. Other alternative embodiments are possible, as will be evident to those of ordinary skill in the art having the benefit of this disclosure.

Referring to FIG. 6C, system 642 includes three clusters 644, 646 and 648 coupled together by system transmission lines 650, 652 and 654. Each node may be configured as previously described. This last configuration is shown to provide yet another example of the variety of configurations in which the present invention may be employed. Any number of nodes in any numbers of clusters may be combined, so long as each nondriving node is terminated.

An alternate embodiment contemplated by the inventors for the systems of FIG. 2 and FIG. 4 also terminates a transmission line using either a driver in a termination mode, or using an on-chip terminator, but the configuration of each is changed.

In previous embodiments described herein, a terminating driver included either a pull-up or pull-down circuit having an output resistance matching the characteristic impedance of the transmission line. One of either the pull-up or pull-down circuit would be enabled for a period of time when it was desired that the transmission line be terminated. In this embodiment, pull-up and pull-down circuits are designed to be simultaneously enabled when the driver is in a termination mode.

In situations where a terminator is employed, two possibilities exist. The terminator may include upper and lower resistance elements such as the pull-up and pull-down circuits described with respect to terminating drivers and which terminate to the upper and lower rails respectively, or may instead include a resistance element which terminates to a voltage V_(TERM), the resistance element having a resistance matching the characteristic impedance of the transmission line.

Where a driver is employed to terminate, the driver includes a pull-up circuit having an output resistance and a pull-down circuit having an output resistance, with the values for the output resistances being chosen so that the parallel combination of the pull-up and pull-down output resistances be matched to the characteristic impedance of the transmission line. Thus,

(R _(PU) *R _(PD))/(R _(PU) +R _(PD))=Z _(O)  (R.11)

In systems using drivers to terminate transmission lines at one or more nodes by simultaneously enabling their pull-up and pull-down drivers, it is preferred but not required that all drivers in the system used for termination of a transmission line have substantially similar pull-up and pull-down circuit output resistances, in addition to meeting the requirements of relationship (R.11)

Those of ordinary skill in the art having the benefit of this disclosure will appreciate that when the pull-up and pull-down circuits are simultaneously enabled to terminate the transmission line to which these circuits are connected, the transmission line is properly terminated, and no reflections will occur at that node.

Enabling pull-up and pull-down circuits simultaneously has the effect of coupling a resistance matching the characteristic impedance of the transmission line between the transmission line and a voltage V_(TERM) given by

V _(TERM) =V _(ss)+(V _(DD) −V _(SS))(R _(PD) /[R _(PU) +R _(PD)])  (R.12)

where again V_(ss) is the voltage of the lower power rail, V_(DD) is the voltage of the upper power rail, R_(PD) is the output resistance of the pull-down driver, and R_(PU) is the output resistance of the pull-up driver.

In a system having a total of n nodes where all nondriving nodes are terminated using drivers having substantially similar pull-up and pull-down resistances meeting the constraint of relationship (R.11) above, the high and low voltages to which the nondriving nodes settle are

V _(HIGH) =V _(DD)−(V _(DD) −V _(TERM))(R _(PU) /[R _(PU)+(Z _(O) /[n−1])])  (R.13)

and

V _(LOW) =V _(ss)+(V _(TERM) −V _(ss))(R _(PD) /[R _(PD)+(Z _(O) /[n−1])])  (R.14)

where V_(DD) is the upper power rail, and V_(SS) is the lower power rail.

Subtracting relationship (R.14) from relationship (R.13) gives a voltage swing at a nondriving node as

V _(SWING) =V _(HIGH) −V _(LOW)=  (R.15)

 =(V _(DD) −V _(ss))−[(V_(DD) −V _(TERM))(R_(PU) /[R _(PU) +Z _(O) /[n−1])])]−[(V _(TERM) −V _(ss))(R _(PD) /[R _(PD)+(Z_(O) /[n−1])])]]

This approach provides the flexibility to choose not only the voltage swing present at a nondriving end but also to choose where the swing voltages lie between V_(DD) and V_(SS). This flexibility may be desired if the receiver circuits present at one or more nodes operate more advantageously over a certain range of input voltages.

The driver system just described can also be used with the resistor network of FIG. 4. In such a system, the values of the resistance elements are determined using relationship (R.5) as previously described. However, when the resistor network is present, V_(HIGH) and V_(LOW) are given by

V _(HIGH) =V _(TERM)+(V _(DD) −V _(TERM))(Z _(O) /[R _(PU)+(Z _(O) /[n−1])])  (R.16)

and

V _(LOW) =V _(TERM)−(V _(TERM) −V _(ss))(R _(PD) /[R _(PD)+(Z _(O) /[n−1])])  (R.17)

While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims. 

What is claimed is:
 1. An information handling system, comprising: at least a first and second transmission line each having a characteristic impedance, each of said transmission lines having a first end and a second end, each of said respective first ends being commonly connected to each other; a driver circuit connected to said second end of said first said transmission line; at least one receiver circuit connected to said second end of said second transmission line not connected to said driver circuit; and at least one on-chip terminator, each terminating a second end of a different one of each of said transmission lines not connected to said driver circuit, each of said terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull down resistance matching the characteristic impedance of said transmission lines such that an output resistance of said at least one on-chip terminator is approximately within ten percent of the characteristic impedance of the transmission lines.
 2. An information handling system, comprising: at least two transmission lines each having a characteristic impedance, each of said transmission lines having a first end, each of said first ends being commonly connected to each other first end, and a second end; a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver coupled to a second end of a different one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines such that an output resistance of each of said drivers is approximately equal to the characteristic impedance of the transmission lines; and at least one receiver circuit coupled to a second end of one of said transmission lines.
 3. An information handling system, comprising: at least three transmission lines each having a characteristic impedance, each of said transmission lines having a first end, each of said first ends being commonly connected to each other first end, and a second end; a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; at least one receiver circuit coupled to a second end of one of said transmission lines; and at least one on-chip terminator, each of said on-chip terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said terminator pull-up resistance with said terminator pull-down resistance matching the characteristic impedance of said transmission lines such that an output resistance of said at least one on-chip terminator is approximately within ten percent of the characteristic impedance of the transmission lines.
 4. An information handling system, comprising: at least three transmission lines each having a characteristic impedance, each of said transmission lines having a first end, each of said first ends being commonly connected to each other first end, and a second end; a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; at least one receiver circuit coupled to a second end of one of said transmission lines; and at least one on-chip terminator, each of said on-chip terminators having an output resistance matching the characteristic impedance of said transmission lines such that an output resistance of said at least one on-chip terminator is approximately within ten percent of the characteristic impedance of the transmission lines.
 5. An information handling system, comprising: a plurality of resistance elements; at least a first and second transmission Line each having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a driver circuit connected to said second end of said first said transmission line; at least one receiver circuit connected to said second end of said second transmission line not connected to said driver circuit; and a plurality of on-chip terminator each terminating a second end of a different one of each said transmission lines not connected to said driver circuit, each of said terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines such that an output resistance of said at least one on-chip terminator is approximately within ten percent of the characteristic impedance of the transmission lines.
 6. An information handling system, comprising: a plurality of resistance elements; at least two transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver coupled to a second end of a different one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines such that an output resistance of each of said drivers is approximately equal to the characteristic impedance of the transmission lines; and at least one receiver circuit connected to a second end of one of said second transmission lines.
 7. An information handling system, comprising: a plurality of resistance elements; at least three transmission line having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a plurality of drivers, each driver coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines such that an output resistance of each of said drivers is approximately equal to the characteristic impedance of the transmission lines; and at least one receiver circuit connected to a second end of one of said transmission lines.
 8. An information handling system, comprising: a plurality of resistance elements; at least three transmission line having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; at least one receiver circuit connected to a second end of one of said transmission lines; and at lease one on-chip terminator, each of said on-chip terminators having an output resistance matching the characteristic impedance of said transmission lines such that said output resistance is approximately within ten percent of said characteristic impedance of said transmission lines.
 9. A method for operating an information handling system, comprising: providing at least two transmission lines each having a characteristic impedance, each of said transmission lines having a first end, each of said first ends being commonly connected to each other first end, and a second end; providing a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver coupled to a second end of a different one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; at least one receiver circuit coupled to a second end of one of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing each terminating driver to enable its pull-up circuit and pull-down circuit, to terminate the transmission line to which said terminating driver is connected such that an output resistance of each of said terminating drivers is approximately equal to said characteristic impedance of said transmission line to which said corresponding terminating drivers is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected.
 10. A method for operating an information handling system, comprising: providing at least three transmission lines each having a characteristic impedance, each of said transmission lines having a first end, each of said first ends being commonly connected to each other first end, and a second end; providing a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to the second end of one of said transmission lines; providing at least one on-chip terminator, each of said on-chip terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said terminator pull-up resistance with said terminator pull-down resistance matching the characteristic impedance of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing each terminating driver to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said terminating driver is connected; causing at least one on-chip terminator to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said at least one on-chip terminator is connected such that an output resistance of said on-chip terminator is approximately within ten percent of said characteristic impedance of the transmission line to which said terminator is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected.
 11. A method for operating an information handling system, comprising: providing at least three transmission lines each having a characteristic impedance, each of said transmission lines having a first end, each of said first ends being commonly connected to each other first end, and a second end; providing a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; providing at least one on-chip terminator having an output resistance matching the characteristic impedance of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing at least one terminating driver to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said terminating driver is connected; causing at least one on-chip terminator having an output resistance matching the characteristic impedance of said transmission lines to terminate the transmission line to which said at least one on-chip terminator is connected such that said output resistance is approximately within ten percent of said characteristic impedance of the transmission line to which said terminator is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected.
 12. A method for operating an information handling system, comprising: providing a plurality of resistance elements; providing at least three transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; providing a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver coupled to a second end of a different one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing each terminating driver to enable its pull-up circuit and pull-down circuit, to terminate the transmission line to which said terminating driver is connected such that an output resistance of each of said terminating drivers is approximately equal to said characteristic impedance of said transmission line to which said corresponding terminating drivers is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected.
 13. A method for operating an information handling system, comprising: providing a plurality of resistance elements; providing at least two transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; providing a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; providing at least one on-chip terminator, each of said on-chip terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said terminator pull-up resistance with said terminator pull-down resistance matching the characteristic impedance of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing at least one terminating driver to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said terminating driver is connected such that an output resistance of said terminating driver is approximately equal to said characteristic impedance of said transmission line to which said terminating driver is connected; and causing at least one on-chip terminator to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said at least one on-chip terminator is connected such that an output resistance of said on-chip terminator is approximately within ten percent of said characteristic impedance of the transmission line to which said terminator is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected.
 14. A method for operating an information handling system, comprising: providing a plurality of resistance elements; providing at least three transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; providing a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; providing at least one on-chip terminator, each of said on-chip terminators having an output resistance matching the characteristic impedance of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing at least one terminating driver to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said terminating driver is connected such that an output resistance of said at least one terminating driver is approximately equal to said characteristic impedance of said transmission line to which said at least one terminating driver is connected; causing at least one on-chip terminator having an output resistance matching the characteristic impedance of said transmission lines to terminate the transmission line to which said at least one on-chip terminator is connected such that said output resistance is approximately within ten percent of said characteristic impedance of the transmission line to which said terminator is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected.
 15. An information handling system, comprising: a plurality of resistance elements; at least a first and second transmission line each having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a driver circuit connected to said second end of said first said transmission line; at least one receiver circuit connected to said second end of said second transmission line not connected to said driver circuit; and a plurality of on-chip terminator each terminating a second end of a different one of each said transmission lines not connected to said driver circuit, each of said terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; wherein each of said resistance elements has a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of transmission lines in the system.
 16. An information handling system comprising: a plurality of resistance elements; at least two transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver coupled to a second end of a different one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; and at least one receiver circuit connected to a second end of one of said second transmission lines; wherein each of said resistance elements has a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of transmission lines in the system.
 17. An information handling system, comprising: a plurality of resistance elements; at least three transmission line having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a plurality of drivers, each driver coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; and at least one receiver circuit connected to a second end of one of said transmission lines; wherein each of said resistance elements has a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of the transmission lines in the system.
 18. An information handling system, comprising: a plurality of resistance elements; at least three transmission line having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; at least one receiver circuit connected to a second end of one of said transmission lines; and at lease one on-chip terminator, each of said on-chip terminators having an output resistance matching the characteristic impedance of said transmission lines; wherein each of said resistance elements has a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of the transmission lines in the system.
 19. A method for operating an information handling system, comprising: providing a plurality of resistance elements; providing at least three transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; providing a plurality of drivers equaling the number of transmission lines in said plurality of transmission lines, each driver coupled to a second end of a different one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing each terminating driver to enable its pull-up circuit and pull-down circuit, to terminate the transmission line to which said terminating driver is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected; wherein said step of providing said plurality of resistance elements further comprises providing a plurality of resistance elements each having a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of the transmission lines in the system.
 20. A method for operating an information handling system, comprising: providing a plurality of resistance elements; providing at least two transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; providing a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; providing at least one on-chip terminator, each of said on-chip terminators comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said terminator pull-up resistance with said terminator pull-down resistance matching the characteristic impedance of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing at least one terminating driver to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said terminating driver is connected; causing at least one on-chip terminator to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said at least one on-chip terminator is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected; wherein said step of providing said plurality of resistance elements further comprises providing a plurality of resistance elements each having a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of the transmission lines in the system.
 21. A method for operating an information handling system, comprising: providing a plurality of resistance elements; providing at least three transmission lines having a characteristic impedance, each of said transmission lines having a first end and a second end wherein said first ends are coupled together through different ones of said resistance elements; providing a plurality of drivers, each driver individually coupled to a second end of one of said transmission lines, each driver comprising a pull-down circuit having a pull-down resistance and a pull-up circuit having a pull-up resistance, the parallel combination of said pull-up resistance with said pull-down resistance matching the characteristic impedance of said transmission lines; providing at least one receiver circuit coupled to a second end of one of said transmission lines; providing at least one on-chip terminator, each of said on-chip terminators having an output resistance matching the characteristic impedance of said transmission lines; designating one of said drivers as the active driver; designating each driver in said plurality of drivers other than said active driver as a terminating driver; causing at least one terminating driver to enable its pull-up circuit and pull-down circuit to terminate the transmission line to which said terminating driver is connected; causing at least one on-chip terminator having an output resistance matching the characteristic impedance of said transmission lines to terminate the transmission line to which said at least one on-chip terminator is connected; and causing said active driver to drive a signal onto the transmission line to which said active driver is connected; wherein said step of providing said plurality of resistance elements further comprises providing a plurality of resistance elements each having a resistance value which is within ten percent of the result of multiplying the characteristic impedance of the transmission line by (n−2)/n where n is the number of the transmission lines in the system. 